1. Field of the Invention
This invention relates to a high solubility, low viscosity, isoflavone enriched vegetable protein isolate and a process for producing the same.
2. Description of the Related Art
The benefits of soy protein are well documented. Cholesterol is a major concern with consumers throughout the industrialized world. It is well known that vegetable products contain no cholesterol. For decades, nutritional studies have indicated that the inclusion of soy protein in the diet actually reduces serum cholesterol levels in people who are at risk. The higher the cholesterol, the more effective soy proteins are in lowering that level.
Soybeans have the highest protein content of all cereals and legumes. In particular, soybeans have about 40.0 wt. % protein, while other legumes have between 20.0 wt. % and 30.0 wt. % protein, and cereals have between about 8.0 wt. % and 15.0 wt. % protein. Soybeans also contain about 20.0 wt. % oil, and the remaining dry matter is mostly carbohydrates (about 35.0 wt. %). In the soybean, both protein and lipid bodies are contained in the usable meat of the soybean, called the cotyledon. The complex carbohydrate (dietary fiber) is also contained in the cell walls of the cotyledon. The outer layer of cells (the seed coat) makes up about 8.0 wt. % of the soybean's total weight. A typical raw soybean includes approximately 18.0 wt. % oil, 15.0 wt. % soluble carbohydrates, 15.0 wt. % insoluble carbohydrates, 14.0 wt. % moisture and ash, and 38.0 wt. % protein.
In processing, soybeans are carefully selected for color and size. The soybeans are then cleaned, conditioned (to make removal of the hull easier) and cracked, dehulled and then rolled into flakes. The flakes are subjected to a solvent bath that removes the oil. The solvent is removed and the flakes are dried, creating the defatted soy flakes that are the basis of all soy protein products. Despite the large number of products on the market, there are only three types of soy protein products: flours, concentrates, and isolates.
Soy flours are the simplest forms of soy protein, having a protein content of approximately 50.0 wt. %. Soy flours are produced by simply grinding and screening the defatted flakes. This simple processing leaves the soy flour with many of the soybean's characteristics. The lack of processing also makes soy flours highly variable in terms of quality.
Soy flours and grits are still widely produced and are used most often in baked goods, snack foods and pet foods applications where the high flavor profile does not pose a problem. Textured soy flours were an early attempt at simulating or enhancing the texture of meat products. Texturizing does not change the composition of soy flours and reduces the flavor profile only slightly. The primary applications of texturized soy flours are inexpensive meat products or pet foods.
Soy concentrates have at least 65.0 wt. % protein. Soy protein concentrates are made by removing soluble carbohydrate material from defatted soy meal. Aqueous alcohol extraction (60-80% ethanol) or acid leaching (at the isoelectric pH 4.5 of the protein) are the most common means for carbohydrate removal. In both aqueous alcohol extraction and acid leaching, however, essentially all of the protein is rendered insoluble. Protein solubility may be recovered in acid leach products by neutralization. A myriad of applications have been developed for soy concentrates and texturized concentrates in processed foods, meat, poultry, fish, cereal and dairy systems.
Isolates are produced through standard chemical isolation, drawing the protein out of the defatted flake through solubilization (alkali extraction at pH 7-10) and separation followed by isoelectric precipitation. As a result, isolates are at least 90.0 wt. % protein on a moisture-free basis. They are sometimes high in sodium and minerals (ash content), a property that can limit their application. Their major applications have been in dairy substitution, as in infant formulas and milk replacers.
Isoflavones occur in a variety of leguminous plants and oilseeds, including vegetable protein materials such as soybeans. These compounds generally include daidzin, 6″-O-acetyldaidzin, 6″-O-malonyldaidzin, daidzein, genistin, 6″-O-acetylgenistin, 6″-O-malonylgenistin, genistein, glycitin, 6″-O-malonylglycitin, glycitein, biochanin A, and formononetin.
It has recently been suggested that the isoflavones contained in vegetable proteins such as soybeans may inhibit the growth of human cancer cells, such as breast cancer cells, prostate cancer cells and colon cancer cells. In addition, it has been suggested that isoflavones reduce cardiovascular risk factors, for example by reducing the levels of atherosclerosis inducing lipoproteins and low-density cholesterol and by increasing endothelial dependent vasodilation response. Isoflavones are also showing promise in preventing osteoporosis and treating menopausal symptoms.
Isoflavone compounds have been associated with an inherent, bitter flavor in vegetable protein materials such as soybeans. In the commercial production of such protein materials, such as protein isolates and protein concentrates, the focus has been to remove isoflavone compounds. For example, in a conventional process for the production of a soy protein isolate, soy flakes are extracted with an aqueous medium having a pH above the isoelectric point of the protein to solubilize the protein. The extract containing the protein is separated from insoluble fiber materials to provide a protein extract. Most of the isoflavones are solubilized in the extract as well as the protein. The protein is precipitated by acid leaching, i.e., adjusting the pH of the extract to about the isoelectric point of the protein, typically between 4.2 and 4.6 for soy protein, with an acid. The precipitated protein is then separated from the extract. Much of the isoflavones remain solubilized in the extract following separation of the precipitated protein (curd) from the extract; however, some of the isoflavones are usually present in the precipitated curd. After separation of the precipitated protein curd from the extract, the extract and the isoflavones solubilized therein are usually discarded. Any residual isoflavones left in the separated protein are removed by exhaustive washing of the protein to ensure that the taste associated with the isoflavones is not present in the protein. Therefore, these commercial isolates contain essentially no isoflavones.
It is desirable, however, to provide an isoflavone rich protein material, and a process for producing the same, which is suitable for administration in a diet. Such an isoflavone rich protein material can be used to provide the nutritional benefits of the protein and the health benefits of the isoflavones when administered in a diet.
It is known to produce a protein material which includes isoflavones by a process that involves subjecting a vegetable material containing protein and isoflavones to extraction with an aqueous extractant having a pH above the isoelectric point of the protein material to obtain an extract that contains isoflavones and protein. The pH of the extract is adjusted to about the isoelectric point of the protein material to precipitate a protein curd containing isoflavones. The protein curd is separated from the extract at a temperature of about 20° C. to about 32° C., and optionally washed with only a controlled amount of water, such that many of the isoflavones which are in the precipitated protein curd remain in the resulting isolate. However, because most of the isoflavones remain in the extract when the protein is precipitated at the isoelectric point of the protein, and are subsequently discarded along with the extract, the isoflavone content of these isolates is low, typically below about 0.75 mg/g of total dry matter.
Further, processes that involve the use of isoelectric precipitation, such as those discussed above, reduce the water solubility of the proteins, which is measured by the nitrogen solubility index (“NSI”) of the product. In particular, isolates which are produced by processes which involve the use of isoelectric precipitation have an NSI of below about 70%. Isoelectric precipitation and subsequent neutralization also adds minerals to the product, which increases the ash content.
Water solubility of isolates produced using isoelectric precipitation may be increased with heat treatment of the neutralized protein isolate suspension; however, such heat treatment also increases the viscosity of the finished product such that, an increase in the NSI of such products to above 70% by heat treatment also provides a product having a high viscosity, for example, of at least 2,000 centipoise (cp) when reconstituted in water at a 12.0 wt. % solution at a temperature of about 22° C.
Soybeans contain about 0.5 wt. % saponins. Soy saponins have been the subject of investigation since the early 20th century. These compounds consist of a triterpenoid skeleton with various sugar and acetyl moieties. The current consensus is that soyasapogenols A, B and E are true aglycons, while other soyasapogenols are artifacts of hydrolysis conditions. The corresponding glycosides are the so-called “group A saponins”, “group B saponins”, and “group E saponins”, respectively.
Soy saponins have demonstrated anti-mutagenic properties that make them promising agents for cancer prophylaxis. Moreover, group B soy saponins have exhibited pronounced suppressive effects on the replication in vitro of the human immunodeficiency virus (HIV). The chemical structure of soybean saponins is very similar to that of the compound glycyrrhizin, a known anti-viral agent, so soy saponins show promise as building blocks for the synthesis of anti-viral pharmaceutical compounds.
Despite the cultivation and processing of very large quantities of soybeans, at the present time soy saponins are not a significant article of commerce due to the difficulty of isolating and purifying them.
Bowman-Birk Inhibitor Concentrate (“BBIC”) has been shown to exhibit inhibitory activity against the malignant transformation of cells under certain conditions and its administration has been shown to affect various forms of cancer.
In particular, it has been shown that the enzyme-inhibitor described by Bowman (Proc. Soc. Expd. Med., 63:547 (1946)) and Birk et al. (Bull. Res. Council Israel, Sec. A 11:48 (1962) and Biochim. Biophys Acta, 67:326 (1963)), which is found in soybeans and is subsequently referred to as the Bowman-Birk Inhibitor (“BBI”), can prevent, or greatly reduce, radiologically or chemically induced malignant transformation of cells in culture and in experimental animals.
The present invention provides and produces a soy protein isolate that has a high nitrogen solubility index, and a lower viscosity, higher isoflavones content, and higher saponins content than the conventionally available soy protein isolates.